Method for producing magnesium containing cast iron



United States METHOD FOR PRODUCING MAGNESIUM CONTAINING CAST IRON No Drawing. Application March 27, 1953, Serial No. 345,228

Claims priority, application Great Britain March 31, 1952 4 Claims. (Cl. 75-130 The present invention relates to a new magnesiumcontaining addition agent for the treatment of cast iron melts and to an improved process utilizing said addition agent to produce magnesium-containing cast iron.

It is well known that the addition of elemental magnesium to molten iron leads to a very violent reaction and all, or very nearly all, the magnesium is lost. If the magnesium is added as an alloy with, for example, nickel, there is still a reaction, and for a given type of alloy the violence of this reaction increases with the magnesium content of the alloy.

r Now for the reasons explained in U. S. Patent No. 2,485,760 and British Patent No. 630,070, the production of cast iron containing spheroidal graphite as the result of the presence of magnesium is highly advantageous, and in practice the magnesium is at present commonly added as an alloy containing from 14% to 16% magnesium. The violence of the reaction when this alloy is used is not suificient to prohibit its use but is still enough to warrant special precautions being taken and to lead to loss of a substantial proportion of the magnesium in the addition alloy. Moreover, the fact that the magnesium content of the addition alloy is low means that the total amount of alloy which must be added is high, so that the molten iron is chilled to some extent and an undesirable amount of the metal or metals with which the magnesium is alloyed may be introduced into the iron.

1 It has been proposed heretofore to add magnesium nitride to cast iron. However, magnesium nitride has been found to react very slowly with molten cast iron since the rate of decomposition of the compound when added to the molten iron is so slow that it is difiicult to introduce suflicient magnesium into the molten iron to insure the development of spheroidal graphite in castings made from the melt. However, since magnesium nitride is rich in magnesium and contains no other metal it is asan addition agent.

Ith'as now been discovered that when what may be called a promoter is added along with the magnesium nitride, or an'alloy very rich in magnesium and nitrogen, tosmolten cast iron, the decomposition of magnesium nitride is speeded up so that it is no longer difficult to introduce sufficient magnesium into molten iron to insure the development of spheroidal graphite, and yet a quiet magnesium addition reaction is still obtained.

Broadly stated, the addition agent contemplated by the present invention comprises about to about 70% of one or more promoters selected from the group consisting of metallic magnesium, silicon and carbon, with the balance essentially magnesium nitride. The invention also contemplates the addition to a molten gray cast iron bath of amounts of the aforedeseribed agent to provide a small but eflfective retained magnesium content in castings made from the bath.

The highest proportion of the magnesium is retained in the iron, and therefore the least amount of nitrided magnesium or alloy need be added, when both graphite attractive atent O 2,757,082 Patented July 31, 1956 and ferrosilicon are used. The size of the particles of silicon or compound thereof is an important factor, since if these particles are very fine the reaction increases in violence and molten metal may be ejected from the metal bath. Small lumps of ferrosilicon, from /s to /2 inch in size, may advantageously be used. The efiicacy of ferrosilicon as a promoter is roughly proportional to its silicon content, and material containing more than about 70% silicon is preferred.

It is to be understood that the promoter is added simultaneously with the nitrided magnesium or alloy and should be mixed with it. An inocculant may also be used, that is to say, ferrosilicon may be used as a promoter and then a further addition of ferrosilicon may be used as an inocculant.

When silicon is employed as the promoter, preferably in the form of ferrosilicon, the amount of silicon will usually be about 25% to 70% of the agent. When graphite is employed, the amount of graphite will usually beabout 25% to 70% of the agent. When magnesium is employed, it will usually be about 5% to 30% of the agent and will in no case exceed 50% of the agent.

As an example of the quantities employed, an addition consisting of a mixture of nitrided magnesium in fine powder form and ferrosilicon (containing silicon) in 4; inch lumps, in amounts respectively equal to 1.5% and 1.3% by weight of the iron to be treated, was added to a bath of a molten iron which had a carbon content of 3.5%, a silicon content of 1.1% and a sulfur content of 0.02%. The iron was subsequently inoculated with a further 0.5% of the ferrosilicon. The magnesium content in the iron as cast was 0.10%, that is to say, of the magnesium added, about 9% was retained in the iron.

Graphite is a very good single promoter. It is not important to avoid the use of fine particles, but lumps of A to 4; inch in size are advantageous, as they form a protective layer over the nitrided magnesium, produce a reducing atmosphere and retain heat. As an example, an iron containing 3.4% carbon, 1.8% silicon and 0.01% sulfur was melted and treated with 2% of a mixture consisting of equal parts of nitrided magnesium and graphite, inoculated with 0.5% silicon added as ferrosilicon containing 80% silicon, and cast. The casting contained 0.10% magnesium, that is, about 14% of the magnesium added was retained in the iron.

As an example of the use of graphite and ferrosilicon as joint promoters, 2.7% of a mixture consisting of equal parts of nitrided magnesium, graphite and ferrosilicon (containing 80% silicon) was added to a molten iron which contained 3.5% carbon, 1.8% silicon and 0.01% sulfur. The melt was subsequently inoculated with 0.5 silicon added as ferrosilicon. The retained magnesium content of the iron as cast was 0.11%, representing retention in the iron ofabout 16.5 of the magnesium added.

Elementary magnesium or a magnesium alloy may also be used to promote the decomposition of nitrided magnesium or a nitrided magnesium alloy. In other words, the addition agent is a mixture 'of a major proportion of nitrided magnesium or a nitrided magnesium alloy and a minor proportion of elementary magnesium or a magnesium alloy. As an example, nitrided magnesium added in an amount of 1% to a molten iron of low sulfur content reacted very sluggishly, that is to say, was absorbed only slowly by the molten iron, which had to be held for some time before it could be poured. When, however, the 1% of nitrided magnesium was replaced by 0.85% nitrided magnesium and 0.15% magnesium chips intimately mixed together, this mixture was rapidly and smoothly absorbed into the melt. This is surprising in view of the extreme volatiiity of elementary magnesium.

When an alloy rich in magnesium and nitrogen is employed in combination with a promoter in accordance with the present invention, the nitrided alloy may be a nickel-magnesium alloy, e. g., an alloy containing 56% magnesium and 44% nickel, a nickel-iron-magnesium alloy, a copper-magnesium alloy, a copper-iron-magnesium, alloy, a nickel-silicon-magnesium alloy, an ironsilicon-magnesium alloy, an iron-.silicon-magnesium-calci m alloy, a magnesium-silicon alloy, etc. The nitrided materials used according to the invention are preferably in the form of powder.

It is known that some elements are detrimental to the production of the spheroidal form of graphite. Titanium is an element which, though harmless in the small amounts in which it is usually present, is detrimental in larger amounts. The detrimental effect of titanium is counteracted to some extent by the addition of the magnesium as nitrided magnesium or alloy, possibly because of reaction between the titanium and the nitrogen to give a harmless titanium compound. For example, titanium was deliberately added to an iron which on analysis was found to contain 3.4%, carbon, 1.8% silicon and 0.01% sulfur, and the iron was so treated with a nickel-magnesium alloy and then inoculated with ferrosilicon that the retained magnesium content was 0.07%. Ignoring the titanium, the graphite in the iron as cast should therefore have been spheroidal. It was found, however, that the graphite was in flake form, the titanium content being 0.22%. The same iron, on being treated with 1% nitrided magnesium mixed with 1% graphite and inoculated, with ferrosilicon, had a retained magnesium content of 0.1% and was found tocontain about 95% of the graphite in the spheroidal form. Moreover, the titanium content had fallen to 0.16%.

The agents contemplated in accordance with the present invention may be prepared in the form of briquettes with or without other materials, e. g., the briquettes may contain up to about 75% nickel powder, up to about 75%, iron powder, or up to about 75% of copper powder, the balance being the addition agent. Binders may be used to provide solidity in such briquettes. In preparing briquettes, it is preferred to mix the ingredients thoroughly before, pressing. The agents contemplated by the pres ent invention, with or without the aforesaid additional materials, can, of course, be used in other forms, e. g., as. a mixture, etc.

It is desirable when small but effective amounts of magnesium, e. g., about 0.02% or 0.03% to about 0.4%., are introduced into molten gray cast iron by means of the special addition agent provided in accordance with the, present invention that the magnesium-containing molten. iron be thereafter inoculated with a graphitizing inoculant, e. g., silicon, ferrosilicon, calcium silicide, etc,., and be, cast in an inoculated condition to retain thev aforesaid amounts of magnesium in the final castings. In, this manner, graphite in the castings is induced to occur in a compacted form, especially a spheroidal form. Inoculants are usually added in amounts of about 0.3% to about 2.5% of the melt treated.

The cast iron compositions treated in accordance the invention will usually contain about 1.7%- or 2% or 2.5%. up, to about 4% or 5% carbon, about 0.4%; or 1% or 1.5% up to about 4% or 6% silicon, with the; balance essentially iron, said balance usually being about 85%or 87% or more of the castings. The molten compositions treated in accordance with the invention may also contain the usual alloying elements found in gray cast irons, white cast irons and malleable irons. The impurities phosphorus and sulfur may be present in amounts of up to 0.25% and 0.3%, respectively, but it is preferred that the contents of these impurities be below about 0.15%, respectively. The invention is particularly applicable to the production of products containing a small but eifective amount up to about 0.4% magnesium with the balance essentially a gray cast iron composition.

Although the present invention has been described in conjunction with preferred embodiments, it is. to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. The process for introducing magnesium. into molten cast iron to produce magnesium-containing iron castings which comprises adding to molten cast iron magnesium in the form of an agent comprising a mixture of magnesium nitride, silicon and carbon, with the silicon and carbon being present in the amount of about 25% to about of said mixture, and thereafter casting the resulting magnesium-containing molten cast iron in an inoculated condition.

2. The process for introducing magnesium into molten cast iron to produce magnesium-containing iron castings which comprises adding to molten cast iron magnesium in the form of an agent comprising a mixture of magnesiumnitride and silicon containing about 25% to about 70% silicon, and thereafter casting. the magnesium-containing molten cast iron in an inoculated condition.

3. The process. for introducing magnesium into molten cast iron to produce magnesium-containing iron castings which comprises adding to molten cast iron magnesium in theform of an agent comprising a mixture of magnesium nitride and carbon containing about 25% to about 70% carbon, and thereafter casting the magnesium-containing molten cast iron in an inoculated condition.

4. The process for introducing magnesium into molten cast iron to produce magnesium-containing iron castings which comprises adding to molten cast iron magnesium in the formof an agent comprising a mixture of about 25% to about 70% of an element selected from. the groupconsisting of silicon and. carbon with the balance assert-- tially magnesium, nitride, and thereafter casting. the magnesium-containing molten cast iron.

References Cited in the file of this patent lished in 1912. Subject: Beitrage Zur Kennis Der Metallnitride. 

1. THE PROCESS FOR INTRODUCING MAGNESIUM INTO MOLTEN CAST IRON TO PRODUCE MAGNESIUM-CONTAINING IRON CASTINGS WHICH COMPRISES ADDING TO MOLTEN CAST IRON MAGNESIUM IN THE FORM OF AN AGENT COMPRISING A MIXTURE OF MAGNESIUM NITRIDE, SILICON AND CARBON, WITH THE SILICON AND CARBON BEING PRESENT IN THE AMOUNT OF ABOUT 25% TO ABOUT 70% OF SAID MIXTURE, AND THEREAFTER CASTING THE RESULTING MAGNESIUM-CONTAINING MOLTEN CAST IRON IN AN INOCULATED CONDITION. 